Good points all of them, but there's something else to keep in mind and the water pump analogy points it out well. It's not always just about max efficiency. Any engine has certain conditions where it really performs. For the big block V8 its low rpm heavy load conditions. For my little D15 honda engine, it's light load, moderate rpm (3000 or so) and for my 2.3L cosworth mercedes, it's heavy load, high rpm (6000 plus). None of these engines are guaranteed to be more fuel efficient than the others except within the conditions where they work the best.
Unless you drive under a very narrow set of circumstances (Over the road truckers do that, but few other drivers do) you're going to be moving in and out of the most efficient range of the engine in question. You can either try to adjust the car and the engine so that the max efficiency occurs in the range of the most common driving conditions, or you can try to widen the efficiency range of the engine to cover more driving conditions (but like a torque curve on a dyno graph, you'll end up with lower peak efficiency over a wider rpm range). You can do both of these things with a turbo depending on how you build the system and size the turbo.
The problem of a turbo either spinning up early and running out of gas, or not spooling until late in the RPM range is a dead issue. The turbochargers designed within the last decade are leaps and bounds ahead of what most people realize and are used to. The old standby garret t series turbos are such old technology at this point that I think they should be scrapped altogether by the garage tuner crowd. They have major performance limitations compared to the new generation turbos which can perform over a wide variety of driving conditions, rpms, and throttle loads.
In terms of the small engine vs. large engine in the same car. You're only presenting half of the argument. Put a 4 cylinder into a chevy pickup and it's not likely to get any better mileage than a V6. But the V6 chevy pickups definitely get better gas mileage than the V8s... (and in case you haven't driven a full size pickup with a chevy V6, I have. Believe me they have just as much get up and go as the V8 models.) Similarly, if you put a V8 in a honda, I'm willing to bet you won't get better gas mileage than the stock 4 cylinder. Ditto for a big 6 cylinder. All this is because to move a certain mass at a certain speed it's going to take (setting things like gear ratio and wind resistance aside) a specific amount of power. Change the mass you move, and you change the power needs and the best engine for the job. If you can accept that premise, then you can move on to how to extract the needed power in the most efficient manner.
Diesels inherrently get better gas mileage, and basically run on knock/detonation. This allows them to run much more agressive turbo setups that produce tons of intake temp heat with little to no negative effect.
Diesel and Gasoline turbo setups are apples to oranges.
And I agree that if you're only ever using 40% of your engine, then you could probably survive with a smaller displacement engine and get better efficiency out of it.
But realistically for production cars they can't go much smaller. They can, but not much. Safety standards make it extremely hard to make a light AND affordable car, and with curb weight averages where they're at it's hard to use a small engine that still makes people happy.
The big downside to using smaller engines though, is that the difference in MPG will be quite obvious as you add passangers, drive around the city, or drive over steep hills. You'll be working that engine pretty hard, or going painfully slow. I'd assume the people here might be ok with going slower to not push the smaller engine too hard and keep efficiency, but the average driver will just drive harder and get worse gas mileage than if they had a larger engine.
I think the factory Civic VX was a very good balance of economy and power. Even the CRX HF was pretty good. Plenty of torque for the weight, and due to it's small nature it would be hard to add too much weight to kill the mileage for that power range.
Now, if you put a 1 liter engine in that same car, the torque is going to be severely hindered. Adding a turbo would then be REQUIRED to have any efficiency under acceleration or load.
That's not a bad idea, I never said it was. I'm just still trying to figure out for myself weather or not adding a turbo to an already efficient setup will actually help.
Oh, there was some talk about the costs to try this out by HAL9000. I might beat him to it if things go smoothly the next 6 months. I'll have megasquirt installed for under $1000 in the next 4 months or so (maybe closer to $600, but I'm budgeting high). Once that's done I can tune it and see what mileage I get on the stock T04B 40 trim. Then I can upgrade to a T04E 46 trim which seems perfect for getting some mild boost on the freeway and still supporting enough airflow for my goals. Those turbos run about $100 used, and rebuild kits are cheap.
As far as I can tell HAL, we completely agree upon the engine size discussion.
The reason to have a V8 in a truck though, is that when towing the V6 will NOT be happy, and I HAVE done a fair amount of towing. It's amazing that a F-250 with a 460 Ford can get nearly identical mileage unloaded as it does under a 2000# tow. Most of the difference mileage is aerodynamic. A V6 with similar HP would be severely compromised by that load.
I had a 1 liter metro that got around 46 mpg about 10 years ago (I didn't care about mileage back then) it weighed about 1800 pounds. did it bogg when I had 4-5-6 people in it? yes!!! I actually had 7 in it at one time (back when my friends were all little). the fact is that you don't buy THAT car to haul people.
I have been playing around with the idea of getting a wildfire. it has a brisk 26HP motor. if they made it with a little more tail and could go 65 easily, I would be sold on the car. that is more my point. there is a market out there for these cars if they were to make one with say 40 or so HP and could go highway speeds. of course I wouldn't haul many people (if any) with me when I drove it. also it weighs about 700 lbs. (been a while since I checked, I may be wrong). it gets away with it because it is a 3 wheeler and is considered a motorcycle.
the 40 hp I am talking about could easily be achieved with a small turbo on maybe a slightly larger motor and the speed could be done with gearing.
I do also agree that there won't be much advantage (if any) to adding a turbo to an existing car. especially if you take into consideration the cost of the turbo setup.
Be the change you wish to see in the world
Can't go tooooo much smaller though. My Rebel will only get 65mpg on the freeway. That's a 250cc engine around peak torque and 85-90% throttle. Peak torque is about 7000 and that's doing around 70. So that engine is running as efficiently as possible on the freeway and only manages that. The engine is maxxed out around 80 on a flat road.
In town it is more than enough and gets in the high 90s but it's completely gutless otherwise.
The way I drive my VX, it could easily survive on a 1 liter engine with supercharging, or turbocharging for the very small percentage of time when I actually need more than 30-40 horsepower. I seldom exceed 2500 RPM in a whole tank of fuel.
If Alfa Romeo could get 390 horspepower out of a 1500 CC enigne in 1950, how much more power do you want in a 2000 pound car. I think 300 HP out of 2.5 liters would be fine. Consider the Honda S2000 at 245 out of 2 liters without supercharging.
How many horsepower do you need to pull a trailer? Most of it is used to accelerate the total weight, then the power demand drops off quite a bit when all you have to do is maintain speed.
Hypermiling itself realizes the fact that almost without exception cars are overpowered. The less percentage of peak power output needed to maintain a highway speed, the less efficient the vehicle will be as far as mileage.
Get the CD down to .19, a totally practical figure, and the power necessary to maintain speed drops, but so does the percentage of power (compared to best efficiency) and mileage will not increase significantly without reducing engine displacement to increase percentage of load.
Stop thinking about the engine itself as the source of power. Capacitive storage of energy, with indinitely variable transmissions, will allow a vehicle to accelerate to 60 MPH in less than 5 seconds, regardless of the engine size. Small percentage peak power demands should be provided by stored energy, not some hugely oversized engine.
Think of it this way, you have a guage that instantly gives you the exact power your engine is producing, and gives you a graph of the time period of operation.
A turbo give you the ability to use a 2.5 liter 4 cylinder engine in a pickup truck, and haul a load. You just need the infinitely variable tranny, and the turbo to get you up the mountain, the only place where sustained demand approaches maximum power output for any significant period of time.
Sure you wont be able to climb a 10% grade at 80 MPH, but for twice the mileage I would be willing to make that sacrifice.
Our views are definitely only a few degrees apart, but that's the nice thing about polite conversations. You have the opportunity to really get into all the nitty gritty little details.
Personally, I think you could do the MS conversion (I'm assuming that it's on the 280) for more like $400. I picked up a broken MS box and repaired it for under $100 including the cost of a stimulator to test it. Fuel injectors, rails, and harnesses can be had on ebay for a song, as can throttle bodies, O2 sensors, and all the other parts you need. Enough people have done conversions on the old datsun straight sixes that you should be able to find the "grassroots recipe" on the msefi forum. I put all the parts together for my mercedes (except for the manifolds which I have to custom fabricate this winter) for under $1250. That's to convert the car to efi and build a custom turbo system AND modify the engine internals to put up with the roughly 125 hp/cylinder that it'll be producing.
Regarding how small the manufacturers can go and keep the public happy. Look at the cars in the UK and other European countries, India, Japan, China, and the like. That's the direction that we're going to end up going whether we like it or not. There's a reason why the average fuel economy in the US is in the low teens but in the high 40s in Europe. The fact is that Americans are behind the times in terms of average fuel efficiency because we've never paid the "real" price for transportation at the pump. Most European countries build their transportation/road taxes into the price of gas, so it becomes a real pay-per-use tax and people can see what it costs to drive a mile. If America did that I think the shock would make many of us change our habits substantially. (Or we'd lynch the local legislators!)